1. Field of the Invention
Embodiments of the invention are applicable to any data communications equipment with one or more redundant copies of components. For example, embodiments of the invention can be used to switch the active data path from one copy to another without introducing any bit errors.
2. Description of the Related Art
Synchronous optical networking (SONET) and synchronous digital hierarchy (SDH) networks have been widely deployed as the primary optical network in the telecommunications infrastructure globally. In this disclosure, SONET terminology is used. Since SDH networks have largely equivalent terms, the concepts disclosed are easily converted to SDH equipment. The basic SONET STS-1 transport frame is shown in FIG. 1. Each frame includes 9 rows and 90 columns of bytes, and is transmitted once every 125 microseconds, at a bit rate of 51.84 Mbps. The first three columns are the transport overhead—TOH bytes. The remaining 87 bytes are the Synchronous Payload Envelope—SPE. Bytes are generally labeled by their offset relative to the H1, H2 pointer bytes. The payload frame has 9 rows and 87 columns of bytes. The first column of the payload frame is the Path Overhead—POH bytes. The payload frame begins at the J1 bytes and floats within the transport frame and can, therefore, straddle two transport frames. The beginning of the transport frame is located by the H1, H2 pointer bytes.
A number of STS-1 streams can be multiplexed together to form a higher rate signal. The constituent STS-1 streams are first frame aligned to a common transport frame alignment by an STS payload processor (shown in FIG. 2), which includes a Pointer Interpreter, a FIFO buffer, and a Pointer Generator. Then, the aligned STS-1 streams are byte interleaved to form an STS-N stream. The number of STS-1 streams that can be interleaved to form an STS-N stream is limited to N=3, 12, 48, 192, and 768. The resultant STS-N stream has a bit rate that is N times that of an STS-1 stream. It should be noted that the constituent STS-1 streams are typically independent and can have arbitrary alignment and slightly different bit rates. Consequently, a SONET multiplexer tends to establish an independent bit rate for the resultant STS-N stream. The payload processor uses STS-1 pointer justifications to adapt between the incoming STS-1 alignment and rate to the resultant STS-N alignment and rate. Often, the decision of what type (positive or negative) pointer justification and which frame to insert it, is based on the current depth of the FIFO associated with each constituent STS-1 stream.
An STS-1 stream can be further sub-divided into a number of virtual tributaries—VT. Four virtual tributaries types are defined, VT1.5, VT2, VT3 and VT6. The frame format of a virtual tributary is shown in FIG. 3. The bytes labeled V1 to V4 are VT transport overhead bytes and bytes V5, J2, Z6 and Z7 are VT path overhead bytes. As with STS-1 frames, the payload frame of a VT floats within a VT transport frame and is located by the V1, V2 pointer bytes. Multiplexing virtual tributaries together into an STS-1 SPE typically requires the constituent VTs to be aligned to a common VT transport frame alignment by a VT payload processor. The VT payload processor also converts incoming STS-1 and VT pointer justifications into a combined set of VT pointer justifications. As with STS-1 pointer processors, the decision of what type (positive or negative) of pointer justification and which frame to insert it, is often based on the current FIFO depth of each constituent VT stream. The structure of a VT payload processor (VTPP) is logically identical to that of an STS payload processor (shown in FIG. 2).
FIG. 4 shows a typical piece of SONET equipment; a SONET network element (SONET NE). Traffic is carried by two independent sets of fibers. In a Linear 1+1 Protection scheme, or a Unidirectional Path-Switch Ring (UPSR), the fibers carry identical traffic and are called the working fiber and the protect fiber. In a Bidirectional Line-Switched Ring (BLSR), portions of the bandwidth in each fiber are reserved to carry Protection traffic. In the event of one of the fibers or the associated framer experiencing a failure, the SONET equipment can send and receive data using the other fiber and framer. Schemes that protect against fiber and framer failures are called facility protection schemes. Due to the large differences in distance in the path taken by the working path and the protect path, switching between the two paths hitlessly typically requires large amounts of memory to compensate for the relative delays in arrival times. This problem of facility protection is the subject of many issued patents. See, for example, U.S. Pat. No. 4,477,895 to Casper, et al.; U.S. Pat. No. 4,686,675 to Morimoto, et al.; U.S. Pat. No. 5,051,979 to Chaudhuri, et al; U.S. Pat. No. 5,285,441 to Bansal, et al.; U.S. Pat. No. 5,577,196 to Peer, et al.; U.S. Pat. No. 5,631,896 to Kawase, et al.; U.S. Pat. No. 5,825,821 to Okuyama; U.S. Pat. No. 6,246,668 to Kusyk; U.S. Pat. No. 6,515,962 to Sawey, et al; U.S. Pat. No. 6,754,172 to Tanaka, et al; and U.S. Pat. No. 6,795,393 to Mazzurco, et al.